The technology explosion in the implantable medical device (IMD) industry has resulted in many new and innovative devices and methods for analyzing the health of a patient and/or providing therapies to improve quality of life. IMDs include pacemakers, implantable cardioverter-defibrillators (ICDs), neural stimulators, drug administering devices, monitors, etc. State-of-the-art IMDs are capable of performing significantly more complex tasks and are vastly more sophisticated and complex than earlier IMDs and their therapeutic benefits have been well established.
Cardiac IPGs and monitors as well as other IMDs are powered by an internal power source, typically one or more batteries, to serve a variety of functions, including, but not limited to, supplying power to electronic components and circuitry and charging high voltage capacitors that are discharged through medical electrical leads into the heart to regulate heart rhythms. The functional sophistication and complexity of the IMD operating systems powered by the power source have increased over the years.
IMDs that are powered by a non-rechargeable power source such as a standalone battery must be replaced when the battery becomes depleted, and therefore conserving the energy in the power source is important to maintain or prolong the life of the IMD. Yet, current drain in a medical device comes from several sources. The “static current” is the current required to run the IMD circuitry (including micro-processor). The “stimulation therapy current” (such as pacing pulse current) is the energy required to deliver therapy. In an IMD, capacitors are charged up in anticipation of delivering a stimulation therapy and the energy in the capacitor is delivered if the stimulation therapy is warranted. However, an intervening sensed event will preclude delivery of the stimulation therapy. Depending on the duration over which the stimulation therapy energy is stored in a capacitor, some of the stored energy will bleed-off from the capacitor. As a result, a significant amount of the power source may be lost from the charged capacitor even without delivery of a stimulation pulse. Therefore, conservation techniques and circuitry are needed to reduce the energy losses associated with storage of stimulation therapy energy in an implantable medical device.